1. Field of the Invention
This invention relates to hard-disk data storage using longitudinal patterned recording media and more particularly to apparatus and methods for orienting magnetic anisotropy in longitudinal patterned recording media.
2. Description of the Related Art
Conventional longitudinal magnetic recording media generally comprises granular magnetic layers formed by sputtering Co-alloys onto a suitable aluminum alloy or glass substrate. The magnetization of conventional longitudinal media is typically in the plane of the disk. Data is written onto the media by applying a localized magnetic field using a recording head that glides over the surface of the media.
The areal density of conventional magnetic storage media has been increasing at more than one hundred percent annually. This increase has typically been achieved by reducing the dimensions of the magnetic grains while continuing to store information on a conventional granular magnetic medium using in-plane magnetization. However, estimates show that thermal energy starts to compete with the anisotropy energy per grain if grain volumes are continually reduced. If the volume of the grains is reduced too much, the magnetization of the grains becomes unstable and the grain magnetization may flip spontaneously, thereby erasing the data stored thereon. This effect is known as the superparamagnetic effect. To avoid the superparamagnetic effect, the product of the grain volume (V) and the anisotropy energy (Ku) must be maintained above a specified value to keep the individual grains stable.
Since the grain volume (V) must be reduced to provide higher recording densities, one method for maintaining the thermal stability of the magnetic grains is to increase Ku. However, a Ku that is too high results in a media coercivity that is too high. That is, although the magnetic grains would be thermally stable, it would be impossible to write data onto the media because it would require a magnetic field stronger than can be supplied by currently available write heads.
To avoid the limits of the superparamagnetic effect, one possible solution is the use of patterned media. In patterned media, tracks of discrete magnetic islands are formed in a circumferential direction around the media surface. Each island stores a single bit and functions as a single-domain switching volume. Each island may comprise a single magnetic grain or several exchange-coupled grains. Because the size of an island typically exceeds the grain size used in conventional longitudinal recording media, the magnetic switching or grain volume (V) is typically large enough to maintain the thermal stability of each island.
Most studies of patterned media have focused on perpendicular recording applications where the magnetic anisotropy of the media is perpendicular to the substrate. Such a transition to perpendicular recording media, however, would require a transition to perpendicular recording technology, including perpendicular recording heads, perpendicular media read channels, and the like. Thus, to take advantage of longitudinal recording technology, it may be advantageous to develop patterned media for longitudinal applications.
One challenge to implementing longitudinal patterned media is orienting the magnetic anisotropy of the individual islands. When the magnetic grains are grown or deposited on an isotropic media substrate, the magnetic anisotropy (i.e., easy axis) of each grain is randomly oriented in the plane of the substrate. As a result, the magnetic anisotropy of each island is also oriented randomly in the plane of the substrate. This can decrease the signal-to-noise ratio when reading from the islands since the magnetization of the islands is typically measured along the track. If the axis of magnetic anisotropy of an island is oriented perpendicular to the track, a read head will detect little or no signal from the island. Likewise, if the axis of magnetic anisotropy is oriented at forty-five degrees with respect to the track, the signal in the read head will be reduced.
Accordingly, apparatus and methods are needed for orienting the magnetic anisotropy of islands in longitudinal patterned recording media. More particularly, apparatus and methods are needed for orienting the magnetic anisotropy of islands in a circumferential direction around the longitudinal patterned media. Such apparatus and methods are disclosed herein.